Electromagnetic system utilizing multiple pulse transmitter waveforms

ABSTRACT

The present invention provides a transmitter for an electromagnetic survey system for transmitting signals having a waveform comprising at least a first pulse and a second pulse, wherein the first and second pulses are different in at least one of shape and power. Embodiments of the invention enable combining various distinct pulses that may have been optimized for respective applications to form a transmitter waveform for conducting a geological survey. In effect, the embodiments of the present invention provides an EM system that is substantially equivalent to multiple EM systems operating at the same time for collecting data in relation to different aspects of the geology of interest. Advantageously, the benefits of the present invention can be obtained without the undesirable complexity and cost associated with the simultaneous deployment of multiple EM systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/432,614, filed Mar. 31, 2015, which is a National Stage of PCTApplication No. PCT/CA2013/000878, filed Oct. 15, 2013, which is relatedto, and claims priority from, U.S. Provisional Patent Application Ser.No. 61/713,105 filed Oct. 12, 2012, the disclosure of which isincorporated here by reference.

FIELD OF THE INVENTION

The present invention relates to electromagnetic systems, and moreparticularly, to systems and methods for conducting geophysical surveysusing multiple pulse transmitter waveforms.

BACKGROUND OF THE INVENTION

Electromagnetic (EM) measurement systems for geophysical measurementpurposes detect the electric and magnetic fields that can be measuredin, on or above the earth, to identify subsurface changes in electricalproperties of materials beneath the earth's surface. Airborne EM systemscarry out field measurements in the air above the earth. A primary goalis to make measurements at a number of spatial locations to identify thesize and position of localized material property changes. Such changescan be attributed to a desired outcome such as identifying a localizedmineral deposit, a buried object, or the presence or absence of water.

Generally speaking, EM systems usually include a source ofelectromagnetic energy (transmitter) and a receiver to detect theresponse of the ground.

EM systems can be either frequency-domain or time-domain. Both types ofsystems are based on principles encapsulated in Faraday's Law ofelectromagnetic induction, which states that a time-varying primarymagnetic field will produce an electric field. For airborne systems, theprimary field is created by passing a current through a transmitter loop(or series of transmitter loops). The temporal changes to the created orradiated magnetic field induce electrical eddy currents in the ground.These currents have an associated secondary magnetic field that can besensed, together with the primary field, by a series of receiver coils.

Each receiver coil may consist of a series of wire loops, in which avoltage is induced proportional to the strength of the eddy currents inthe ground and their rate of change with time. Typical receiver coilshave axes in the three Cartesian directions that are orthogonal to oneanother. Coils with their axes perpendicular to the earth are mostsensitive to horizontal layers and half-spaces. Coils with their axeshorizontal are more sensitive to discrete or vertical conductors.

In frequency-domain systems, the time-varying transmitter signal is asinusoidal waveform of constant frequency, inducing electrical currentsin the ground of the same frequency. Most systems use several constantfrequencies that are treated independently. Although the secondary fieldhas the same frequency as the primary field, it will have a differentamplitude and phase.

For time-domain systems, a time-varying field is created by a currentthat may be pulsed. The change in the transmitted current induces anelectrical current in the ground that persists after the primary fieldis turned off. Typical time domain receiver coils measure the rate ofchange of this secondary field. The time-domain transmitter currentwaveform repeats itself periodically and can be transformed to thefrequency domain where each harmonic has a specific amplitude and phase.

Existing prior art EM systems have limitations in surveying variousterrains and geologies. For example, time domain EM systems existing inthe prior art are typically configured or optimized to measure aparticular type of terrain or geology near an estimated depth, based ona number of considerations pertinent to each task. These time domain EMsystems generally are not well-equipped to deal with surveys of complexgeology, which may comprise a mixture of deep and shallow geologicalstructures, and/or strong and weak conductivities.

As a result, in some surveys of complex terrain or geology whereexisting prior art time domain EM systems were used, the geology ofinterest would be flown over multiple times, each with a specificallyconfigured EM system for detecting one specific aspect of the terrain orgeology. While this approach may provide desirable survey resolution, itis generally time consuming and not cost-effective.

In some ground-based EM systems existing in the prior art, such as theGeonics™ EM-37 system, two or more transmitter waveforms were used tocollect shallow and deep ground information, wherein repeated pulses ofa first waveform are transmitted and measured, followed by thetransmission and measurement of repeated pulses of a second waveform. Incontrast, for an airborne EM system which is constantly moving, it maybe difficult to use a dual waveform system to collect responses fromboth waveforms over the same geology, which results in poor surveyresolution for the combined waveform survey data.

Therefore, there remains a need for an improved EM surveying system thatcan efficiently and cost-effectively provide measurements of variousterrains and complex geologies.

SUMMARY OF THE INVENTION

The present invention overcomes the above drawbacks of the prior art EMsystems by providing an electromagnetic survey transmitter fortransmitting signals having a waveform comprising multiple pulses thatare different in shape and/or power, and providing an airborne EM systemthat is configurable to transmit signals having a waveform comprisingmultiple pulses that are different in shape and/or power.

The present invention improves the overall resolution of measurementsfrom pulses of the transmitter waveform, and provides advantages indefining geology. The present invention provides benefits that arecomparable to those achievable using multiple simultaneously deployed EMsystems, without the undesirable integration complexity and the costassociated therewith.

In accordance with one aspect of the present invention, there isprovided an electromagnetic survey system, comprising: a transmitter fortransmitting signals having a waveform comprising at least a first pulseand a second pulse, said first pulse and second pulse are different inat least one of shape and power; and a receiver for measuring responsesfrom said signals.

In accordance with another aspect of the present invention, there isprovided a transmitter for an electromagnetic survey system fortransmitting signals having a waveform comprising at least a first pulseand a second pulse, said first pulse and second pulse are different inat least one of shape and power.

In accordance with another aspect of the present invention, there isprovided a method of conducting geological survey, comprising:transmitting signals having a waveform comprising at least a first pulseand a second pulse, said first pulse and second pulse are different inat least one of shape and power; and measuring responses from saidsignals.

Preferably, the transmitter waveform cycles are repeated without overlapin time.

Preferably, the receiver of the electromagnetic survey system measuresthe responses from waveform cycles during and after each (or all)pulse(s) of the waveform have been transmitted.

Other features and advantages of the present invention will becomeapparent from the following detailed description and the accompanyingdrawings, which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the present inventionare described hereinafter with reference to the accompanying drawings,wherein:

FIG. 1 is a diagram of one configuration of an EM system in accordancewith prior art;

FIG. 2 is a block diagram of an example embodiment of an EM system;

FIG. 3 shows an example of a dual waveform in accordance with prior art;

FIG. 4 shows an example waveform in accordance with an embodiment of theairborne EM system;

FIG. 5 shows an example waveform in accordance with an embodiment of theairborne EM system;

FIG. 6 shows an example waveform in accordance with an embodiment of theairborne EM system; and

FIG. 7 is a schematic perspective view of an illustrative embodiment ofthe airborne EM system in an airborne position flying at surveyingspeeds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to theaccompanying drawings, in which some, but not all embodiments of theinvention are shown.

Illustrated in FIG. 1 is an airborne EM system known in the art, whereinthe primary field is created by passing a current through a transmitterloop (or series of transmitter loops). The temporal changes to thecreated or radiated magnetic field induce electrical eddy currents inthe ground. These currents have an associated secondary magnetic fieldthat can be sensed, together with the primary field, by a series ofreceiver coils.

The present invention may be implemented as an EM system such as the oneshown using block diagrams in FIG. 2.

Referring to FIG. 2, the EM system comprises a transmitter section 10,which may include a signal generator 30, and a receiver section 20. Theconfiguration, construction and operation of the receiver 20 and theassociated receiver coils can be provided in accordance withconventional EM practice known to a person skilled in the art.

The present description provides an EM survey transmitter 10 fortransmitting signals having a waveform comprising multiple pulseswherein at least two pulses are different in shape, power, or both.

In accordance with an example embodiment of the present disclosure, theelectromagnetic transmitter section 10 is configurable to transmitsignals having a waveform comprising at least a first pulse and a secondpulse, wherein the first and second pulses are different in at least oneof shape and power.

Referring to FIG. 3, in ground-based EM system existed in the prior art,such as the Geonics™ EM-37 system, dual transmitter waveforms were usedto collect shallow and deep ground information, wherein repeated pulsesof a first waveform are transmitted and measured, followed by thetransmission and measurement of repeated pulses of a second waveform.

In contrast, referring to FIG. 4, an example waveform 32 in accordancewith the present disclosure comprises multiple or a plurality of pulsesincluding at least a first pulse 34 and a second pulse 36, wherein thefirst pulse 34 is different from the second pulse 36 in shape, and/orpower.

As shown in FIG. 4, an EM waveform cycle 38 comprises more than onetransmitted waveform 32 comprising pulses 34 and 36. Repeated waveforms32 in a waveform cycle 38 can be the same or opposite polarity. Eachpulse 34 and 36 of waveform 32 has an ON time period followed by an OFFtime period. Measurements from pulses 34 and 36 are taken in the ON timeand the OFF time of each pulse 34 and 36 with respect to transmittedwaveform 32.

As illustrated in FIG. 4, signals having the waveform 32 can berepeatedly transmitted and the ground responses measured during the ONand OFF time periods of the EM waveform cycles. Namely, multipledistinct pulses are combined within EM waveforms and multiple waveformswithin cycles of pulsed signal transmission, and responses thereof aremeasured in respective EM cycles.

In some embodiments, waveforms 32 comprising multiple distinct pulsesare transmitted in at least some EM cycles of the entire survey task.This allows a mixed use of the multi-pulse waveform 32 disclosed hereinand the conventional pulsed signals to conduct survey over a widevariety of terrains including simple geology, complex geology, or amixture thereof.

The plurality of pulses within a waveform 32 may have variouscharacteristics including shape, amplitude, phase, frequency, andpolarity, and may be arranged in any suitable order or any suitablemanner to form the waveform 32 as described herein.

For example, as shown in FIG. 5, waveform 32 may comprise bi-polarpulses (34 a, 34 b) and (36 a, 36 b) that are different in at least oneof shape and power.

In some embodiments, at least one pulse of waveform 32 can be repeatedwithin an EM cycle 38 of the waveform 32.

Referring to FIG. 6, an example waveform 32 in accordance with thepresent disclosure comprises distinct pulses (for example, 34, 36, and39) that have different ON times or base frequencies, and includespulses having a stepped pulse shape 34. A person skilled in the artwould appreciate that various other pulse shapes can also be used informing the waveform 32, including irregular, triangular, saw-tooth,square, sinusoidal shapes or the like or any combination of any numberof the above.

Preferably, signals formed from various waveforms 32 disclosed hereinare transmitted by the transmitter 10 at a predetermined repetitive ratefor the duration of at least some EM cycles of a survey task, enablingthe collection of consistent data over the entire survey area. However,a person skilled in the art would appreciate that waveform 32 can betransmitted in any fashion suitable for a particular application,including in a random manner, at a constant repetition frequency, or ata variable repetition frequency.

Preferably, at least two of the distinct pulses 34 and 36 of a waveform32 have the same repetition rate.

In some embodiments, the EM transmitter 10 transmits waveforms 32wherein the distinct pulses 34 and 36 have different repetition rates orbase frequencies.

Advantageously, transmitting signals using waveforms 32 disclosed hereinprovides substantially simultaneous measurements of the earth responsefrom multiple distinct pulses 34 and 36 that may be optimized forvarious aspects of a survey task, including deep and shallow geologicalstructures, strong and weak conductivities, and/or variations in extentand orientation of the survey targets. For example, the pulses havinghigher power can be used to detect deeper geological structures, andhigher frequency pulses can be used to detect shallower geology.

Preferably, the delay between the starting times of consecutive cycles38 of waveform 32 is configurable in accordance with the needs of aparticular survey task. In other words, the repetition rate or basefrequency of waveform 32 is configurable.

Advantageously, the repetition rate of waveform 32 can be configured toallow response data collection at the highest possible resolution fromall distinct pulses included in waveform 32.

A preferred delay between transmission of the adjacent pulses 34 and 36of the waveform 32 can be configured so that the EM system would havemoved a small distance during each consecutive pulse 34 and 36.Therefore the resolution of data measured from the pulses 34 and 36 ismuch higher comparing to that of the prior art systems and methods,thereby providing a significant advantage in defining geology.

For example, if an airborne EM system is conducting a survey at 30 Hzrepetition rate, then the preferred delay between pulses 34 and 36 of awaveform 32 can be configured to be less than 16.6 ms and morepreferably 10 ms or less, so that the movement of the EM system betweenthe pulses 34 and 36 of the waveform 32 would be minimized or renderednegligible, thereby enabling substantially simultaneous pulsetransmission and measurement thereof. Therefore the resolution of datameasured from pulses 34 and 36 is much higher comparing to that of theprior art systems and methods, thereby providing a significant advantagein defining geology. The delay between pulses 34 and 36 could be longeror shorter when surveying at a lower or higher repetition rate or basefrequency.

The embodiments of the present invention thus enable combining variouspulses 34 and 36 that are optimal for respective applications to form atransmitter waveform 32 for conducting a geological survey.

In effect, the embodiments of the present invention provides an EMsystem that is substantially equivalent to multiple EM systems operatingat the same time for collecting data in relation to different aspects ofthe geology of interest.

Advantageously, the benefits of the present invention can be obtainedwithout the undesirable complexity and cost associated with thesimultaneous deployment of multiple EM systems.

The embodiments disclosed herein therefore provide an EM system that isconfigurable to transmit signals having a waveform comprising multiplepulses that are different in shape and/or power.

In some embodiments, the EM system comprises a transmitter 10 fortransmitting signals having a waveform 32 comprising at least a firstpulse 34 and a second pulse 36, wherein the first and second pulses aredifferent in at least one of shape and power; and a receiver 20 formeasuring responses from the transmitted signals.

Preferably, the signals having waveform 32 are transmitted bytransmitter 10 without overlap in time. In other words, the beginning ofa cycle 38 b of waveform 32 follows the end of a preceding cycle 38 a ofwaveform 32, as shown in FIG. 6.

Preferably, receiver 20 of the EM system measures the responses fromwaveform 32 during and after each pulse of waveform 32 that istransmitted.

The waveform 32 disclosed herein can be generated using one or moresignal generator 30, which comprises circuitry means for generatingelectrical currents, shaping and adjusting the currents as a function ofthe various parameters of the EM system into pulses, and means forcombining the pulses into a desirable waveform 32.

EM transmitter 10 for transmitting signals in waveform 32 as disclosedherein can be used in various EM survey systems, and in particular inairborne EM systems. For example, in accordance with the presentdisclosure, a method of conducting geological survey comprisestransmitting EM signals having a waveform comprising at least a firstpulse and a second pulse, the first and second pulses being different inat least one of shape and power; and measuring responses from saidsignals.

In some embodiments, the EM system described herein comprises a signalprocessor or means for processing the EM data. In particular, the signalprocessor extracts measurements corresponding to the distinct pulses ofthe transmitted waveform 32, and interprets the correspondingmeasurements in light of all measurements to estimate distribution ofconductivity in the subsurface and various aspects of the geology ofinterest.

A person skilled in the art would appreciate that the above describedinvention may be used in any electromagnetic system using pulsetransmitter waveforms where applicable and is not strictly restricted toan airborne EM system.

For illustrative purpose only, the general configuration of an airborneEM system is described. It is to be expressly understood that thedescription and drawings are only for the purpose of illustration and asan aid to understanding, and are not intended as a definition of thelimits of the invention.

For example, referring FIG. 7, an aircraft towed EM survey systemgenerally comprises a tow assembly 2 comprising a transmitter section 10and a receiver section 20. The aircraft can be manned or unmanned powerdriven fixed-wing airplane, helicopter, airship or any other flyingmachine, as a person skilled in the art would appreciate. Thetransmitter section 10 may comprise a transmitter loop frame whichsupports a transmitter loop coil for generating a primaryelectromagnetic field that induces a secondary electromagnetic field inthe ground. The transmitter frame may comprise tubular sections 12 thatare serially connected at a plurality of joints 14 as shown in FIG. 7.However, a person skilled in the art would appreciate that the systemsdisclosed herein may work with any type of transmitter or generator as asource of electromagnetic energy.

In some embodiments, the transmitter frame comprises tubular sections 12that are made of generally rigid or semi-rigid material. For example,materials such as carbon fiber reinforced plastic, carbon fiberreinforced polymer, unplasticized polyvinyl chloride (uPVC),wood/plastic composite, or any other composite or materials that providestrong rigidity, stability and resistance to deformation, can be used toconstruct tubular sections 12 or portions thereof.

In some embodiments, lightweight materials are used for constructingtubular sections 12 or transmitter section 10 to allow for constructingsizable transmitter frame without significantly increasing the weightthereof.

Using rigid material to construct the transmitter section 10 may allowits size to be increased while maintaining its overall stability andstructural integrity.

In some embodiments, the tubular sections 12 are connected in a mannerthat substantially eliminates the relative rotation between theconnected tubular section 12, thereby allowing the transmitter frame toretain a rigid shape during operation, or preventing distortion of theshape of the transmitter section 10.

The rigid and modular transmitter frame 10 provides stable support forlarge transmitter loop and will maintain its rigidity and stability asthe size of the transmitter loop varies. For example, transmitter loophaving diameter in excess of about 30 meters and weight of about 500 kgcan be achieved.

The receiver section 20 of the example embodiment shown in FIG. 7 ispositioned along a central axis that is substantially perpendicular tothe plane defined by the transmitter frame, and is coupled to thetransmitter section 10 by a plurality of cross support means 4 such ascross ropes or cross bars or rods. However, a person skilled in the artwould appreciate that the systems disclosed herein may work with anytype of receiver section 20 in other suitable configurations. Forexample, the receiver section 20 may be disposed in a co-planar fashionwith the transmitter section 10, or may be concentric or co-axial withthe transmitter section 10. The receiver section 20 may be positionedabove, within, or below the plane as defined by the transmitter section10, at the center of the transmitter section 10, and/or offset from thecenter of the transmitter section 10. The receiver section includes areceiver which may be supported in any manner known in the art and maycomprise at least one receiver coil.

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, otherembodiments and modifications are possible. Therefore, the scope of theappended claims should not be limited by the preferred embodiments setforth in the examples, but should be given the broadest interpretationconsistent with the description as a whole.

1. An electromagnetic survey system, comprising: a signal generator; atransmitter connected to the signal generator and configured to produceelectrical current signals having a waveform comprising at least a firstpulse and a second pulse, said first pulse and second pulse aredifferent in at least one of shape and power; and a receiver formeasuring responses induced by said electrical current signals, whereinan amplitude of each of the first and second pulses continuously changesduring at least a portion of an ON time period, and wherein the firstpulse is sandwiched by two second pulses and the second pulse issandwiched by two first pulses.
 2. The electromagnetic survey system ofclaim 1, wherein said waveform is transmitted at a predeterminedrepetition rate.
 3. The electromagnetic survey system of claim 1,wherein said first pulse and second pulse have the same repetition rate.4. The electromagnetic survey system of claim 1, wherein: theelectromagnetic survey system comprises an airborne electromagneticsystem conducting an electromagnetic survey at a predeterminedrepetition rate; and the transmitter sets a delay between the firstpulse and the second pulse of the waveform based on the predeterminedrepetition rate so that movement of the airborne electromagnetic systembetween the first pulse and the second pulse is rendered negligible andsimultaneous pulse transmission and pulse measuring is enabled.
 5. Theelectromagnetic survey system of claim 4, wherein: the predeterminedrepetition rate comprises 30 Hz; and the delay is less than 16.6 ms. 6.A transmitter for an electromagnetic survey system for transmittingsignals, the transmitter comprising: a signal generator; and one or morecoils connected to the signal generator and configured to produceelectrical current signals having a waveform comprising at least a firstpulse and a second pulse, said first pulse and second pulse beingdifferent in at least one of shape and power, wherein an amplitude ofeach of the first and second pulses continuously changes during at leasta portion of an ON time period, and wherein the first pulse issandwiched by two second pulses and the second pulse is sandwich by twofirst pulses.
 7. The transmitter of claim 6, wherein said waveform istransmitted at a predetermined repetition rate.
 8. The transmitter ofclaim 6, wherein said first pulse and second pulse have the samerepetition rate.
 9. A method of conducting geological survey,comprising: producing, with one or more coils, electrical currentsignals based on a signal generator, the electrical current signalshaving a waveform comprising at least a first pulse and a second pulse,said first pulse and second pulse are different in at least one of shapeand power; and measuring responses induced by said electrical currentsignals, wherein an amplitude of each of the first and second pulsescontinuously changes during at least a portion of an ON time period, andwherein the first pulse is sandwiched by two second pulses and thesecond pulse is sandwich by two first pulses.
 10. The method of claim 9,wherein said waveform is transmitted at a predetermined repetition rate.11. The method of claim 9, wherein said first pulse and second pulsehave the same repetition rate.
 12. The method of claim 9, wherein:producing electrical current signals further comprises transmitting anelectromagnetic waveform cycle comprising repeated transmissions of thewaveform; and the first pulse and the second pulse are different inshape in each repeated transmission of the waveform.
 13. The method ofclaim 9, wherein: producing electrical current signals further comprisestransmitting an electromagnetic waveform cycle comprising repeatedtransmissions of the waveform; and the repeated transmissions of thewaveform in the electromagnetic waveform cycle comprise a bi-polar pulseand at least one pulse that is not a bi-polar pulse.
 14. The method ofclaim 9, wherein: producing electrical current signals further comprisestransmitting a plurality of electromagnetic waveform cycles; eachwaveform cycle in the plurality of electromagnetic waveform cyclescomprises multiple waveforms; and each waveform comprises a plurality ofdistinct pulses having at least one of different on times and basefrequencies.
 15. The method of claim 14 wherein: at least one waveformin the multiple waveforms comprises the first pulse and the secondpulse; and measuring responses further comprises measuring responsesduring on time periods and off time periods of the first pulse and thesecond pulse in each one of the plurality of electromagnetic waveformcycles.
 16. The method of claim 15, wherein producing electrical currentsignals further comprises transmitting the at least one waveform in onlysome of the plurality of electromagnetic waveform cycles.
 17. The methodof claim 9, wherein: the first pulse and the second pulse of thewaveform each comprise the ON time period followed by an OFF timeperiod; and measuring responses further comprises measuring responsesfrom each one of the first pulse and the second pulse during the ON timeperiod and the OFF time period of each one of the first pulse and thesecond pulse.
 18. The method of claim 9, wherein the first pulse and thesecond pulse comprise bi-polar pulses.
 19. The method of claim 10,wherein: producing electrical current signals further comprisestransmitting signals from an airborne electromagnetic system conductingan electromagnetic survey at the predetermined repetition rate; and themethod further comprises setting a delay between the first pulse and thesecond pulse of the waveform based on the predetermined repetition rateso that movement of the airborne electromagnetic system between thefirst pulse and the second pulse is rendered negligible and simultaneouspulse transmission and pulse measuring is enabled.
 20. The method ofclaim 19, wherein: the predetermined repetition rate comprises 30 Hz;and the delay is less than 16.6 ms.